Supernova Relic Neutrino Background Research Articles

Review of uncertainties in the cosmic supernova relic neutrino background

We review the computation of and associated uncertainties in the current understanding of the relic neutrino background due to core-collapse supernovae, black hole formation and neutron star merger events. We consider the current status of uncertainties due to the nuclear equation of state (EoS), the progenitor masses, the source supernova neutrino spectrum, the cosmological star formation rate, the stellar initial mass function, neutrino oscillations, and neutrino self-interactions. We summarize the current viability of future neutrino detectors to distinguish the nuclear EoS and the temperature of supernova neutrinos via the detected relic supernova neutrino spectrum.

Neutrino signatures from the first stars

Evidence from the WMAP polarization data indicates that the Universe may have been reionized at very high redshift. It is often suggested that the ionizing UV flux originates from an early population of massive or very massive stars. Depending on their mass, such stars can explode either as type II supernovae or pair-instability supernovae, or may entirely collapse into a black hole. The resulting neutrino emission can be quite different in each case. We consider here the relic neutrino background produced by an early burst of Population III stars coupled with a normal mode of star formation at lower redshift. The computation is performed in the framework of hierarchical structure formation and is based on cosmic star formation histories constrained to reproduce the observed star formation rate at redshift z \la 6, the observed chemical abundances in damped Lyman alpha absorbers and in the intergalactic medium, and to allow for an early reionization of the Universe at z \sim 10-20. We find that although the high redshift burst of Population III stars does lead to an appreciable flux of neutrinos at relatively low energy (E_\nu \approx 1 MeV), the observable neutrino flux is dominated by the normal mode of star formation. We also find that predicted fluxes are at the present level of the SuperK limit. As a consequence, the supernova relic neutrino background has a direct impact on models of chemical evolution and/or supernova dynamics.

Neutrino decay and the supernova relic neutrino background

Future experiments will be able to observe the neutrinos coming from all the past supernovae in the universe, the so-called Supernova Relic Neutrino Background (SRNB). These neutrinos have travelled from cosmological distances before reaching the Earth. The measure of this background would be possible in the next generation of neutrino detectors. We show that the measure of the SRNB flux could give information on very long-life neutrino decays, in particular for “invisible decays”, such as ν decay into Majoron.

The supernova relic neutrino backgrounds at KamLAND and Super-Kamiokande

We calculate the supernova relic neutrino (SRN) background flux for the KamLAND and Super-Kamiokande (Super-K) detectors, motivated by the reduction in background at Super-Kand new results for the star formation history (e.g., from theSloan digital sky survey (SDSS)). Our best estimate for theflux at Super-K is slightly below, but very close to, the current Super-K upper limit. The Super-K upper limit isalready inconsistent with a range of star formation historiesallowed by the SDSS data. We estimate that the SRN backgroundshould be detected (at 1σ) at Super-K in a total ofabout 9 years (including the existing 4 years) of data. WhileKamLAND is a much smaller detector compared to Super-K, itprofits from being practically background free and from its sensitivity to the lower energy supernova neutrinos.KamLAND could make a 1σ detection of the SRNs with atotal of about 5 years of data. Given the small expected SRNevent rate, we also consider the detection of the SRNs in amodified Super-K detector with a lower threshold and reducedbackground where the time to detection can be reduced by afactor of ten relative to the existing Super-K estimate.

Detectability of the supernova relic neutrinos

We investigate the flux and the event rate of the supernova relic neutrino background (SRN) at the Super-Kamiokande (SK) detector for various neutrino oscillation models with parameters inferred from recent experimental results. A realistic model of neutrino emission from supernova explosions are adopted in the calculation. The number flux over entire energy range is found to be ∼ 11 cm −2 s −1. We discuss the detectability of SRN at SK, comparing this SRN flux with other background neutrinos in more detail than previous studies. Even though there is no energy window in which SRN is dominant, we might detect it as the distortion of the other background event. We found in the energy range 17–25 MeV the expected event rate at SK 0.4–0.7 yr −1. In this range, ten-year observation might enable us to detect SRN signal (at one sigma level) in the case of LMA solar neutrino solution.

Supernova relic neutrino background

An upper bound to the supernova relic neutrino background from all past type II supernovae is obtained using observations of the universal metal enrichment history. We show that an unambiguous detection of these relic neutrinos by the SuperKamiokande detector is unlikely. We also analyze the event rate in the Sudbury Neutrino Observatory (where coincident neutrons from ${\overline{\ensuremath{\nu}}}_{e}\stackrel{\ensuremath{\rightarrow}}{D}{\mathrm{nne}}^{+}$ might enhance background rejection), and arrive at the same conclusion. If the relic neutrino flux should be observed to exceed our upper bound and if the observations of the metal enrichment history (for $z<1)$ are not in considerable error, then either the type II supernova rate does not track the metal enrichment history or some mechanism may be responsible for transforming ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu},\ensuremath{\tau}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}.$

EFFECTS OF NEUTRINO OSCILLATION ON THE SUPERNOVA RELIC NEUTRINO BACKGROUND

We investigate to what extent the oscillation or conversion of neutrinos enhances the expected event rate of the supernova relic neutrino background (SRN) at the SuperKamiokande detector (SK). The SRN [Formula: see text] can be almost completely exchanged with vμ-like neutrinos by the MSW oscillation under the inverse mass hierarchy with Δm2~ 10−8–105[eV2], or by the magnetic moment of Majorana neutrinos with μv≳10−12μB and Δm2~10−4–10° [eV2]. In the standard calculation of the SRN flux, the event rate of the SRN [Formula: see text] at the SK in the observable energy range of 15–40 MeV can be enhanced from 1.2 yr−1 to 2.4 yr−1 if all [Formula: see text] are exchanged with vμ-like neutrinos. The enhancement is prominent especially in the high energy range (≳ 25 MeV). In the astrophysically optimistic calculation, the event rate becomes as high as 9.4 yr−1. Because the theoretical upper bound of the SRN events without oscillation is about 5 yr−1 taking into account the various astrophysical uncertainties, we might have to resort to the neutrino oscillation if more than 5 events in a year, as well as a significantly harder spectrum, were observed in the SK.

Spectrum of the Supernova Relic Neutrino Background and Evolution of Galaxies

The spectrum of the supernova relic neutrino background (SRN) from collapse-driven supernovae ever occurred in the universe is calculated by using a realistic, time-dependent supernova rate derived from a standard model of galaxy evolution based on the population synthesis method. The SRN spectrum we show here is the most realistic at present, because the largest uncertainty in previous theoretical predictions has come from unrealistic assumptions of the supernova rate so far made. The SRN is one of the targets of the Superkamiokande (SK) detector which will be constructed in a year and the SRN, if at all detected, would provide a new tool to probe the history of supernova explosions in the universe. The expected event rate at the SK is therefore calculated in this paper. Our major results include: (1) the supernova rate is much higher in the early phase of evolution of galaxies and there appears a hump in the SRN spectrum in the low-energy region of $\ltilde 5$ MeV, (2) the SRN flux depends on the Hubble constant ($H_0$) in a way approximately proportional to $H_0^2$ and only weakly on the density parameter of the universe ($\Omega_0$) and a cosmological constant ($\lambda_0$), and (3) the plausible event rate at the SK is 1.2 yr$^{-1}$ in the observable energy range. Such a low event rate is due mainly to a quite low supernova rate at present which is averaged over the morphological types of galaxies. The most optimistic rate in our model is found to be 4.7 yr$^{-1}$, and if more events are detected, we will have to reconsider our current understanding of collapse-driven supernovae and evolution of galaxies.